In scanned detector systems such as television cameras or laser scanners, there usually exists the problem that when viewing a perfectly white target, the resulting video signals do not necessarily appear flat as they should, but contain various amounts of shading. This is the result of various factors, such as change in the lens transmission across its field of view, camera tube beam landing errors, or optical system misalignment. This shading represents an error in the signal and results in the inaccurate reproduction of the scenes viewed. Unfortunately, the sources of the errors cannot be eliminated easily. In television cameras various adjustable signals such as ramps and parabolas in both scan directions are made available to be added or subtracted from the video signals so as to produce a flat signal when viewing a white scene.
The detection system of the present invention is characterized in that the detected video level signal is compared to a reference level and the video level signal is corrected for an output flat response.